Combination drill bit



A. B. HILDEBRANDT 3,066,749 COMBINATION DRILL BIT Dec. 4, 1962 3 Sheets-Sheet 1 Filed Aug. 10, 1959 Alexander B. Hildebrondi Inventor By 9 M...) Z. M Attorney Dec. 4, 1962 A. B. HILDEBRANDT 3,066,749 COMBINATION DRILL BIT Filed Aug. 10, 1959 3 Sheets-Sheet 2 Alexander B. Hildebrondi Inventor By $00M.) 5 Qu-L Attorney Dec. 4, 1962 A. B. HILDEBRANDT 3,066,749

COMBINATION DRILL BIT Filed Aug. 10, 1959 3 Sheets-Sheet 3 Alexander B. Hildebrundt Inventor By 2%) 2. Q i Attorney United States Patent ()filice fifibdfi-Q Patented Dec, 4-, 1%32 ware Filed Aug. Ill), 1959, Ser. No. 832,539 14 Claims. Cl. l'75317) The present invention relates to methods and apparatus for drilling boreholes in the earth and more particularly relates to an improved bit for drilling oil wells, gas Wells and similar boreholes which is provided with dual cutting elements adapted to permit drilling in both hard and soft formations with greater effectiveness. In still greater particularity, the invention relates to an improved rotary drill bit provided with a shoe bearing a cutting element adapted for drilling hard subterranean formations and having extensible cutting blades confined by the shoe which are particularly adapted for drilling in softer formations.

This application is a continuation-in-part of application SN. 796,708, filed March 2, 1959.

A majority of the oil wells, gas wells and similar boreholes produced in recent years have been drilled by rotary drilling methods. Such methods in general involve the use of a bit which is rotated against the formation being drilled into in order to produce a combination of scraping, grinding and percussive forces which break dowh the formation beneath the bit relatively rapidly. Rotation of the bit is usually accomplished by clamping the upper end of a string of drill pipe extending from the bit to the earths surface in a rotar table and rotating the entire drill string. In order to remove the cuttings produced by the action of the bit, a drilling fluid is injected downwardly through the drill string into the bottom of the borehole and is withdrawn through the annular space between the drill string and the borehole wall. A variety of rotary bits of different design, including roller bits, drag bits, ring bits and the like, may be used in such rotary drilling operations.

A substantial portion of the cost of a rotary drilling operation is attributable to the fact that drill hits new employed must be replaced at very frequent intervals. in moderately hard, abrasive formations for example, a conventional drag or roller bit may drill from about 200 to about 300 feet before it must be replaced. Since oil and gas wells and similar boreholes are often drilled to depths of from 10,600 to 15,000 feet or more, the cost of the bits used is generally substantial. A further expense arises from the fact that in a rotary drilling opera tion the entire string of drill pipe must be pulled from the bOTZrO1 each time that the bit has to be replaced. Pulling the drill string is a time-consuming operation and becomes progressively more expensive as the depth of the borehole increases. In drilling operations carried out at great depths, the cost of changing bits may greatly overshadow the cost of the bits themselves, although both factors are of prime importance in any rotary drilling operation.

It is therefore a, primary object of the present invention to reduce present rotary drilling costs by providing 1ong-life drill bits having dual cutting elements which require replacement less frequently than do conventional bits. it is also an object of the invention to provide drill bits having dual cutting elements which are adapted to drill in hard and soft rmations with equal effectiveness. It is a further object of the invention to provide long-life drill bits which are simple and inexpensive in construction and which are adapted for use with conventional drilling equipment. A still further object of the invention is to provide bits having extensible cutter blades whose cutting surfaces are automatically renewed as they become worn.

These and related objects which will become apparent in the following description are realized in accordance with the invention through the use of rotary drill bit provided with a shoe bearing a cutting element adapted for drilling through extremely hard formations and having one or more extensible cutter blades mounted within and confined by the body of the bit. The extensible cutting blades, used in drilling through soft and moderately hard formations, are advanced automatically with respect to the body of the bit and hence continuously present fresh cutting surfaces to compensate for wear which occurs during drilling. The combination of a cutting element particularly adapted for use in drilling hard formations and extensible cutter blades which can be continuously renewed for drilling in softer formations results in a rotary drill bit which has a greater useful life than conventional bits employed in the past and therefore permits drilling for much longer sustained periods than has been possible heretofore.

The nature and objects of the invention can be more fully understood by reference to the following detailed description of the construction and use of bits embodying the invention and to the attached drawing in which:

FIG. 1 is a longitudinal, partially-sectioned view of a preferred embodiment of the invention FIG. 2 is a section FIG. 1;

FIG. 3 is a sectional of FIG. 1;

FlG. 4 is a sectional view taken along the lines 4-4 of FIG. 1;

FIG. 5 is a sectional of FIG. 1;

FIG. 6 is a bottom KG. 1;

FIG. 7 is a longitudinal, partially-sectioned view of a further embodiment of the invention;

PEG. 8 is a sectional view of the bit of HG. 7 taken along the lines 8-8;

FIG. 9 is a sectional view of the bit of FIG. 7 taken along the lines 9-9;

FIG. 10 is a sectional view of the bit of FIG. 7 taken along the lines lltllti;

FIG. 11 is a sectional view of the bit along the lines Jll1l;

FlG. l2 depicts an alternate means of regulating the relative position of the blades and shoe in the bit of FIG. 7; and,

FIG. 13 shows apparatus for of the blades in the bit of FIG. 7.

Turning now to FIGURE 1 of the drawing, it will be seen that the apparatus depicted therein comprises a cylindrical body member 84 arranged in a piston-cylinder relatronship with respect to shoe 85 which supports and protects blade 86. Body member 84 is adapted to be con nected at its upper end to a suitable coupling 87 or to a drill collar or section of drill pipe. The bore 88 of body 34 forms a continuation of the bore 39 of coupling 87 and perm ts the passage of drilling fluid downwardly through the bit apparatus. Body member 8a is open at its lower end and is adapted to receive shoe 85 in a sliding, telescopic relationship.

AS is apparent from the drawing, the upper part of shoe 85 is positioned within the lower portion of body member 84 in a piston-cylinder relationship. 0-ring 9d or a similar sealing device of hemp, rubber, plastic or the like provides a fluid-tight seal between the inner wall of the body member and the outer longitudinal surface of shoe 85. The upper end of the shoe forms a generally cylindrical recess 91 adapted to receive retrievable plug 92. Fluid taken along the lines 22 of view taken along the lines 5-5 view of the apparatus depicted in of FIG. 7 taken preventing overextension view taken along the lines 33 I conduits 93, 94 and 95 communicate between recess 91 and the bottom of shoe and permit the passage of drilling fluid downwardly through the bit into the bottom of the borehole. The total cross-sectional area of the fluid conduits 93, 94 and 95 preferably remains substantially constant over the entire length of the shoe. The conduits emerge from the bottom of the shoe as jet nozzles 96, 9'! and 93 which serve to direct the flow of drilling fluid about the cutting elements on the shoe.

The middle and lower sections of shoe 85 contain longitudinal slot 99 adapted to receive blade 86. The shoe is preferably made in two sections, 100 and 101 in FIG- URES 14 through 18, in order to facilitate insertion of the blade. Screws N2, 1% and 1M may be provided for holding shoe sections 1% and 1M together. The screw heads may be spot-Welded in place after the bit has been assembled in order to prevent their being loosened during operation of the bit.

Blade 36 is an elongated member having a reaming edge Hi5 and a drilling edge 1%. The upper portion of the blade extends upwardly above the reaming edge 105 and is afiixed to body member 84 by screws Hi7 and H38. Vertical slot 109 is provided in the lower portion of the body member to receive blade 86. The blade and body member are thus fixed with respect to one another and cannot move independently. Any downward thrust applied to the body member during drilling will be transmitted directly to the blade. Splines and groove 110 on the shoe and blade permit longitudinal movement of the shoe with respect to the blade, but prevent the blade from being forced laterally out of slot 99 in the shoe. The shoe thus continuously supports the blade and at the same time is free to move in a vertical direction With respect to the blade and the body member 254.

In addition to blade 86 which is employed for drilling in soft and moderately hard formations, the drill bit depicted in FTGURES 1 through 6 of the drawing is provided with cones or rollers lift and 112. These may be of the type conventionally employed on rock bits used in the drilling of oil wells, gas wells, and similar boreholes. The axles and bearings which support the cones on the bottom of the shoe are not shown. Although two cones are depicted in this particular embodiment of the invention, it will be recognized that a greater or lesser number of cones may be employed. in like manner, modifications to permit the use of more than a single cutting edge or blade will be obvious to those skilled in the art. In lieu of the cones shown in FIG- URES 1 and 6, the bottom of shoe $5 may be provided with abrasive inserts, diamond particles or tungsten carbide for example.

The reaming edge res of blade 36 extends parallel to and laterally displaced from the longitudinal axis of the drill bit. The reaming effect of the blade may be enhanced by the insertion of tungsten carbide or diamond particles in a metallic matrix. The blade element itself should be composed of hardened alloy steel in order to produce the most effective cutting action with the least wear. Abrasive inserts on the reamin edge of the blade are shown in FIGURES 1 through 6 and identified by reference numeral 113.

Retrievable plug 92 which is adapted to seat in chamber 91 of shoe S5 is a generally tubular member having a Venturi-like internal configuration. The throat of the plug, designated by reference numeral 114, is reduced in diameter and serves to restrict the flow of fluids through the plug. Openings 115 are provided in the upper portion of the plug to permit the free entry of drilling fluid into the plug. Spear 116 on the top of the plug permits it to be withdrawn from chamber i l by means of conventional overshot or similar fishing equipment. When plug 92 is thus in place, the hydraulic pressure exerted by the drilling fluid in passing downwardly through the bit apparatus is greater than the pressure exerted in the absence .of the plug. This difference in pressure permits Ll precise control of the relative position of the blade and shoe. Plugs having various throat diameters may be employed to give greater or lesser pressure differentials.

In drilling with the apparatus depicted in FTGURES 1 through 6 of the drawing, the bit, attached to a string of rotary drill pipe and associated equipment or similar bit driving means, is lowered into the borehole in substantially the condition shown. Retrievable plug 2 is in place. The upper part of shoe rests in the lower portion of body 84- and is retained therein by the upper part of blade as, on which the upper portion of the shoe rests. In this position, the shoe extends below drilling edge tee of blade lid and protects it. If the bit were lowered into the borehole with the blade distended, the weight of the bit and drill string on the extended blade might lead to blade failure. Axial pressure is applied to the bit by means of the rotary table or other conventional bit driving means and drilling fluid is circulated downwardly through the drill string and bit. Because of the small cross-sectional area of throat lid is retrievable plug 92, the drilling fluid exerts a relatively high hydraulic thrust on shoe $5 and hence the shoe extends below the blade despite the axial force on the bit. The drill string and bit are then rotated from the earths surface. With the bit in this position, drilling takes place primarily as a result of the action of cones Ill and 112. Drilling fluid passing into the bottom of the borehole through nozzles 95, 97 and 98 entrains cuttings produced by the cones and carries them upwardly through the annular space between the bit and the borehole wall.

When a relatively soft formation in which a drag-type bit operates more effectively than a cone bit is encountered, retrievable plug 2 is lifted out of recess 91 in shoe 85 by means of a wire line overshot which grasps Spear 116 on top of the plug. The increase in the bore diameter of the shoe, due to removal of the plug, results in a decrease in the hydrostatic force exerted on the shoe by the downward flowing drilling fluid. As a result of this reduction in hydraulic force or thrust, the blade and body of the bit gradually move downwardly with respect to the shoe. The blade thus projects below the shoe and is forced into the formation below the level at which cones ill and 1112 contact the formation. As the bit rotates, the blade cuts the formation below the cones and hence the bit acts primarily as a drag bit rather than as a cone bit. The portion of the shoe adjacent to the cutting edge of the blade serves to support the blade. As the blade wears during extended drilling, the axial thrust downwardly on the bit from the bit driving mechanism continuously forces the blade downwardly with respect to the shoe. A fresh cutting edge is therefore continuously presented to the formation. The outer, longitudinal edge of the blade serves to ream the borehole and maintain gage.

When it is desired to resume drilling with the cones, retrievable plug 92 is again lowered into recess 91 in the upper portion of the shoe, thereby increasing the hydraulic force or thrust on the shoe. The shoe moves downwardly with respect to the body and blade of the bit so that the cones drill at a level below the cutting edge of the blade and the blade is protected by the shoe. The apparatus of the invention thus permits the use of the blade in formations where drag-type bits are most effective and enables the driller to employ the rotary cones or similar external cutting elements on the lower surface of the shoe for drilling in other formations. This dual cutting action, coupled with the use of an extensible long-life blade, permits drilling for much longer sustained periods than is possible with conventional equipment. The life of the bit of the invention is much greater than that of bits used heretofore and hence the frequency with which the drill string must be pulled in order to change bits is greatly reduced. The effect is a substantial reduction in the overall cost of rotary drilling.

A further embodiment of the invention provided with dual cutting elements and having other means for controlling the relative positions of the blades and shoe is depicted in FIGURES 7 through 13 of the drawing.

Turning now to FIG. 7, the bit apparatus depicted therein comprises a supporting member consisting of upper section 117 and lower sectiontitd. Upper supporting member 117 is provided with threads or other means for attaching the bit apparatus to the lower end of a suitable coupling, drill collar or length of dual pipe represented by reference numeral 119. Upper supporting member 117 and lower supporting member 118 are connected together by means of threads or other suitable connecting means. Disposed within the supporting member and in slidable relationship thereto is a body member consisting of sleeve 1213, intermediate member 121 and shoe 122. Blades 123 and 124 are attached to the lower end of lower supporting member 11%.

Upper supporting member 117 is a generally tubular member defining a central passageway 125 through which drilling fluid may flow downwardly through the drill string into the bit apparatus. Annular groove 126 in the upper part of member 117 serves to limit the upper movement of a retrievable plug 127 in a manner to be explained in greater detail hereafter. The inner diameter of supporting member 117 is enlarged in the lower portion of the member, providing a chamber 125 into which intermediate member 121 may slide. Sleeve 12% is slidably disposed in passageway 125 in the upper part of supporting member 117 and is provided with 0 rings or similar sealing means 129 to prevent the escape of drilling fluid from passage 125 into chamber 128. Port 130 in the outer wall of supporting member 117 provides for the release of pressure within chamber 128.

As pointed out heretofore, lower supporting member 118 is connected to the lower end of upper supporting member 117. Slidably disposed in the lower supporting member and in chamber 128 in the upper supporting member is intermediate body member 121. Spacer 131 serves to center the intermediate body member in chamber 128 and to limit its downward movement in lower supporting member 118. Annular key 132 retains spacer 131 in fixed relationship to member 121. Sleeve 121 is threaded into the central recess in the upper end of member and fluidly communicates therewith.

Shoe 122 is connected at its upper end to the lower end of intermediate member 121 by threads or other suitable connecting means and is provided with slots 13% and 135 which mate with corresponding projections 13d and 137 on the lower portion of lower supporting member 113 to transfer torque between the supporting member and the shoe. The slots and projections are more clearly shown in FIG. 9. Passageways 133, 13?' and 14% in the shoe provide for the transmission of drilling fiuid downwardly from member 121 to nozzles 141 and 1 12 in the bottom of the shoe.

Blade members 123 and 12 1 are supported in slots in the lower edge of supporting member 118 and are retained therein by bolts 143 and 144. The blades are encased in vertical, radial slots in the lower part of shoe 122 in vertically slidable relationship to the shoe. The shoe thus backs up and supports the blades adjacent to the cutting edges. Each blade has a reaming edge 145 extending parallel to the longitudinal axis of the bit and spaced laterally therefrom. Each blade also has a drilling edge 14:: extending across the width of the blade. Only one blade appears in FIG. 7 but the relationship of the blades to one another is clearly apparent from FIGURES and 11 of the drawing. The faces and reaming edges of the blades may be surfaced with tungsten carbide or similar material in order to increase resistance to wear and abrasion. A preferred method for increasing the effectiveness of the blades is to insert bars or rods of tungsten carbide or like material in the blades at spaced intervals as shown in FIGURES 10 and 11 of the drawing. Inserts thus employed may be shrink fitted into holes drilled substan tially the length of the blades. As blades containing such inserts wear during the drilling operation, they gradually assume a fingered contour which is particularly effective. The inserts are indicated by reference numeral 147 in FIGURES l0 and 11.

The lower surface of the shoe 122 is provided with diamond particles or similar abrasive, wear-resistant inserts which serve as the second cutting element of the bit. As shown in H6. 11, these inserts, which are designated by reference numeral 148, may be spaced in a generally spiral pattern behind each blade on the bottom of the shoe. Water courses 149 and 151 are provided to guide the flow of drilling fluid across the surface of the blade into the annular section of the borehole between the bit and the vertical borehole wall. Splines 151 and 152 which seat in slots in the shoe are provided on the backs of the blades to control the lateral postion of the blades with respect to the shoe. As mentioned in connection with the previous embodiment, the slots and splines may be inclined at a slight angle if desired.

Retrievable plug 127, which is shown in FIGURES 7 and 8 of the drawing, is a generally tubular element adapted to seat in the recessed upper portion of sleeve 12% The lower end of the plug is provided with a replaceable nozzle 153 by means of which the size of the orifice in the plug may be varied. The nozzle may be threaded into the lower end of the plug and should be surface hardened to reduce wear and erosion due to the passage of drilling fluid through it. Retractable, springloaded dogs 154 and 155 are provided in the intermediate section of the plug. Spring 156 forces the dogs outwardly against the wall of upper supporting section 117. An individual spring will normally be provided for each dog in lieu of the single spring which is shown in the drawing for convenience. Openings 157, 158, 15? and 160 permit the entry of drilling fluid into the interior of the plug. Spear 161 is provided atop the plug in order to permit its extraction from the drilling apparatus. The barb may be made detachable in order to facilitate removal of the plug from the overshot or similar device employed to pull it out of the apparatus.

Also shown in FIG. 17 is an overshot 162 which may be employed for extracting retrievable plug 127. This device consists in essence of a tubular member adapted to slip down over the plug, depressing dogs 154 and 155, and grasp the spear 161 on top of the plug. Dogs 163 and 164 loaded by springs 165 and 166 are provided in the overshot to prevent the plug from slipping downwardly after it has been grasped. Conduits are provided in the overshot to permit it to be lowered downwardly through a column of drilling fluid. A wire line, not shown, is attached to the top of the overshot to permit it to be raised and lowered. Overshots and similar devices of this general type will be familiar to those skilled in the art. It will be recognized that such overshots are normally longer than that shown in the drawing in order to provide the required weight. It is contemplated that the overshot may be made as long as is necessary.

Drilling is carried out with the apparatus depicted in FIGURES 7 through 11 in much the same manner as described in conjunction with the previous embodiment of the invention. The bit assembly is attached to a string of rotary drill pipe and associated equipment and lowered into the borehole. Retrievable plug 127 may be in place as shown but overshot 162 will not normally be present. As the bit assembly is lowered, the body consisting of sleeve 121), intermediate member 121 and shoe 122 is in a downward position with respect to upper and lower supporting members 117 and 118 and blades 123 and 124. The blades are thus protected against damage which might occur if the assembly were lowered with the blades in an extended, exposed position. Drilling fluid is pumped downwardly through the drill string and the bit assembly is rotated by means of the rotary table or other conventional bit driving means. Because of the small cross section or area in the orifice of nozzle 153 spear/as in retrievable plug 127, the drilling fluid exerts a relatively high hydraulic thrust on the body member and as a result the shoe extends below the blades. With the bit in this position, the action of the blades is restricted to a reaming effect. The abrasive inserts 148 on the bottom of the shoe serve to drill out the bottom of the borehole as the bit is rotated. Drilling fluid passing downwardly through the bit emerges through nozzles 141 and 142 and entrains cuttings produced by the bit, carrying them upwardly through the annular space between the bit and the borehole wall.

To extend the blades for use in a relatively soft formation where a drag-type bit is more effective than a diamond bit, retrievable plug 127 is extracted from the apparatus by means of overshot 162. The overshot is lowered into the apparatus on the end of a wire line until it depresses dogs i and 155 and engages spear 161 on top or the plug. Dogs 7&3 and 164 in the overshot move outwardly beneath the spear and lock the plug and overshot together. The overshot is then lifted out of the borehole and the plug is thus removed. The increase in bore diameter of the apparatus due to removal of the plug results in a decrease in the hydrostatic force exerted on the body member by the downwardly flowing drilling fluid. This reduction in hydraulic thrust causes the blades and supporting member to gradually move downwardly with respect to the body member. The blades of the bit then project beneath the bottom of the shoe and are forced into the formation below the level at which the bottom of the shoe contacts the formation. Rotation of the apparatus leads to a drilling action similar to that of a drag bit. Blade contour is maintained during this drilling action because the inserts 148 on the bottom of the shoe assist in the drilling action at points where particularly hard formations which might otherwise lead to uneven blade contour are encountered. As the blades wear, they continuously move downwardly with respect to the body and member in response to the weight of the drill string applied to supporting members 117 and 11%. Fresh cutting edges are therefore continuously presented to the formation. Gage is maintained by the reaming edges of the blades.

To discontinue use of the blade and resume drilling with the inserts on the bottom of shoe 122., the retrievable plug 127 is dropped into place in the apparatus. The increased by hydraulic force exerted on the body of the bit by the drilling fluid due to the pressure of the plug results in a downward movement of the shoe with respect to the blades of the bit. Drilling with the blades in a retracted position then proceeds.

FIG. 12 of the drawing depicts a still further embodiment of the invention which permits the use of a retrievable plug such as that described above without the necessity for employing a wire-line apparatus for lifting the plug out of the boreholes. In FIG. 12, retrievable plug 167 is shown in place within supporting member 117 resting upon sleeve 120. Dogs 16% and 16% maintained in an outward position by springs 174] and 171 restrict the upward movement of plug and associated components within the supporting member. The plug is extracted from the apparatus by first dropping retriever 172 downwardly in the drill string. The retriever is provided with a grooved projection 173 which mates with the bore of plug 167. Spring-loaded dogs or similar projection 174 and 175 within the plug locked in the groove on projection 173 and tie the plug and retriever together. The retriever is of suflicient length to depress dogs 16% and 169 as ti passes downwardly in the supporting member and to keep them depressed even though plug 167 is its lowermost position. The plug and retriever are then lifted out of the apparatus to the surface by reversing the flow of fluid in the borehole. This device obviates the use of a wire-line retriever and is often preferred as a means for removing the retrievable plugs from the bit apparatus.

A further modification of the apparatus is shown in FIG. 13 and comprises a sleeve 176 provided with con-- duits 177 173 therein. A tubular valve 179 supported by spring 18% is seated in the upper part of the sleeve below retrievable plug 167. The compressibility of spring 18% is such that, in the absence of the plug, the valve is maintained in a position such that passageways 1'77 and 178 are closed off. When the plug is present, the valve is held down so that ports 181 and 182 therein communicate with the passageways. The passageways extend substantially the length of sleeve 176 and emerge therefrom into the annular chamber 12% between supporting member 117 and the outer surface of the sleeve. It should be noted that no port in the supporting member is provided for the release of pressure therein as the body member moves relative to the supporting member.

The modification of the invention shown in FIG. 13 permits the blade and shoe to be held in a fixed position when the apparatus is lowered into or pulled out of the borehole. While drilling with the apparatus, a plug is employed to maintain valve 179 in a lowered, open position. Drilling fluid pumped downwardly through the apparatus can thus pass through passageways 177 and 17% into and out of the annular space defined by supporting member 117 and sleeve 176. Pressure drop through the plug can be regulated for purposes of determining the relative position of a shoe and blade during drilling by employing a replaceable nozzle 153 having the desired orifice diameter. When it is desired to pull the apparatus out of the borehole, the retrievable plug is removed. This causes valve 17? to move upwardly and seal off passageways 177 and 1178. With the valve in this closed position, chamber 123 is a fluid-tight chamber. Movement of the body with respect to the supporting member and blades cannot occur until the valve is again opened by dropping a plug into the bit apparatus. This modification of the apparatus obviates any difficulties which might be occasioned by distension of the blades while lowering the apparatus into the borehole.

-It will be recognized that the apparatus herein described may be utilized in conjunction with down-hole power sources and that many modifications in the apparatus shown in the drawing will be obvious to those skilled in the art. Indexing subs and other apparatus for regulating the size of the orifice in the apparatus of FIGURES 1 through 11 may be employed in lieu of the retrievable plugs shown. Extensible blades may be utilized with various combinations of cones, discs, diamond cutting surfaces, and other external cutting elements positioned on the lower surface of the shoe. Jets, nozzles and water courses in the lower surface of the shoe may be varied in order to effect more efiicient circulation of the drilling fluid between the cutting elements. Other methods for attaching the blade to the body of the bit may be employed. These and other modifications of the apparatus depicted in the drawing will be apparent to those skilled in the art.

What is claimed is:

l. A rotary bit assembly for drilling boreholes in the earth which comprises a supporting member including means for connecting said member to suitable bit driving means;

a first, blade-like cutter element connected to and depending from said supporting member;

a shoe axially slidable with respect to said supporting member and said first cutter element and co-rotatable therewith, said shoe including an upper section of smaller diameter than said supporting member slidably housed within said supporting member and a lower section of larger diameter than said upper section, said shoe containing a longitudinal slot within which said first cutter element extends downwardly below said supporting member;

means within said supporting member above said first cutter element for applying axial thrust to said shoe hydraulically;

and a second cutter element disposed at the lower end of said shoe.

2. A bit as defined by claim 1 wherein said second cutter element comprises abrasive particles supported on said shoe.

3. A bit assembly as defined by claim 1 wherein said second cutter element comprises a rotary cone supported on said shoe.

4. A rotary bit assembly for drilling boreholes in the earth which comprises a supporting member including means for connecting said member to suitable bit driving means, said supporting member containing a longitudinal passageway for transmitting fluids downwardly within said assembly;

a first, blade-like cutter element connected to and depending from said supporting member;

a shoe axially slidable with respect to said connecting means and first cutter element and co-rotatable there with, said shoe including an upper section slidably positioned within said passageway in said supporting member and a lower section of larger diameter than said upper section, said upper and lower sections being slotted radially and longitudinally to receive and confine said first cutter element;

means within said supporting member above said first cutter element for applying axial thrust to said upper section of said shoe hydraulically;

a second cutter element supported on the lower end of said shoe;

and a passageway in said shoe for discharging drilling fluid beneath said shoe, said passageway in said shoe communicating with said passageway in said connecting means.

5. A bit for the rotary drilling of boreholes in the earth which comprises an elongated, hollow supporting member including means for connecting said member to suitable bit driving means;

a blade-like cutter element connected to and depending from said supporting member, said cutter element having drilling and reaming edges;

a shoe extending downwardly from within said supporting member, said shoe being axially slidable with respect to said supporting member and first cutter element, said shoe containing a longitudinal slot within which said first cutter element is confined, and said shoe and supporting member containing a longitudinal passageway for discharging drilling fluid beneath said shoe;

means within said passageway for applying hydraulic thrust to said shoe whereby said shoe moves longitudinally beyond said first cutter element;

and abrasion resistant inserts mounted on the lower surface of said shoe.

6. A bit as defined by claim 5 wherein said means for applying hydraulic force to said shoe includes means for varying the pressure drop through said bit.

7. A bit as defined by claim 5 wherein said inserts comprise diamonds.

8. A bit as defined by claim 5 wherein said inserts comprise particles of tungsten carbide.

9. A rotary drill bit comprising a generally tubular supporting member adapted to be connected to suitable bit driving means;

a body member axially slidable within said supporting member and co-rotatable therewith;

a shoe depending from and co-rotatable with said body member;

a plurality of blades attached to said supporting member and sheathed within longitudinal slots in said shoe;

abrasive inserts on the bottom of said shoe;

passageways within said body member and shoe for the transmission of drilling fluid from said supporting member through said shoe;

and means for moving said body member and shoe relative to said supporting member and blades whereby said shoe extends below said blades in response to hydraulic force in said supporting member.

10. A bit as defined by claim 9 containing a replaceable orifice member whereby movement of said body member and shoe relative to said supporting member and blade is regulated.

11. A bit as defined by claim 9 provided with an annular chamber between said supporting member and said body member and a slidable valve whereby flow of fluid into and from said chamber is regulated.

12. A bit as defined by claim 9 containing a retrievable plug having an orifice therein whereby movement of said body member and shoe relative to said supporting member and blade is regulated.

13. A bit as defined by claim 12 wherein said supporting member and plug are provided with means for limiting upward movement of said body member in said supporting member.

14. A rotary drill bit comprising a tubular supporting member provided with means for connecting said member to the lower end of a drill string, said supporting member containing a longitudinal passageway extending through said member;

an axially slidable, co-rotatable shoe member positioned within said passageway in said supporting member and extending downwardly therefrom, said shoe member including an upper surface within said supporting member against which hydraulic force may be exerted to move said shoe downwardly, a longitudinal passageway extending from said upper surface to a port in the lower surface of said shoe member, and external slots extending longitudinally below said upper surface to the lower surface of said shoe member;

blades connected to said supporting member and extending downwardly within said slots in said shoe member, said blades including longitudinal reaming edges extending laterally beyond said shoe member and lower transverse drilling edges;

means for effecting a seal between said supporting member and said shoe member below said shoulder on said shoe member;

and a plurality of particulate cutting elements embedded in and projecting from the lower surface of said shoe member.

References Cited in the file of this patent UNITED STATES PATENTS 1,394,769 Sorensen Oct. 25, 1921 1,456,350 Widmer May 22, 1923 1,855,330 Zublin Apr. 26, 1932 1,863,928 MacDonell June 21, 1932 1,938,412 Wright Dec. 5, 1933 2,198,849 Waxler Apr. 30, 1940 2,684,835 Moore July 27, 1954 2,693,938 Roberts Nov. 9, 1954 2,740,651 Ortloff Apr. 3, 1956 2,777,523 Brown et a1 Jan. 15, 1957 

